1. Field of the Invention
The invention relates to a self-pumping hydropneumatic spring strut or shock strut with internal level regulation for use with motor vehicles. The shock strut includes a working cylinder which is filled with oil and acted upon with pressure from at least one gas cushion arranged in a high-pressure chamber and acting as a spring. The work cylinder is divided into two work spaces by a work piston having a hollow piston rod and a piston pump which is driven by the spring movements. One of the work spaces is connected to the high-pressure chamber. The piston pump has a pump cylinder formed by the hollow piston rod and delivers oil from a low-pressure chamber to the work space connected with the high-pressure chamber. A hollow pump rod which is fastened to the work cylinder and has a suction valve at its front end penetrates into the hollow piston rod. The bore of the hollow pump rod is connected with the low-pressure chamber. The hollow pump rod also has a regulating opening which is closable as a function of the position of the piston rod in the working cylinder and which connects the work space connected with the high-pressure chamber with the bore of the hollow pump rod.
2. Description of the Related Art
In shock struts of the type mentioned above which are already known, for example, from DE-PS 33 06 393, the work cylinder is divided into two work spaces by a work piston carried by a hollow piston rod and a pump cylinder is formed by the hollow piston rod. A pump rod penetrates into this pump cylinder to form a piston pump. The hydraulic piston pump is constantly in operation during driving operation, so that movements of the vehicle axle and of the piston rod fastened thereto which are brought about by uneven roadways constantly deliver oil to the working cylinder. The piston rod is displaced outward until a regulating opening of the pump rod releases a connection between the work cylinder acted upon by high pressure and the low-pressure space. In this position, the piston pump which continues active does not cause any further outward movement of the piston rod relative to the work cylinder. A disadvantage of this invention is that a pressure compensation takes place between the high pressure and the low pressure space when the regulating opening is released, so that the piston rod dips back into the work cylinder. Therefore, the height or level of the vehicle must then be achieved anew by means of the piston pump.